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Three-phase phase-locked loops (PLLs) are the most widely used synchronization components in power systems. For common PLL algorithms, there is a tradeoff between the filtering capability and dynamic response. In this paper, a nonlinear PLL (NLADRC-PLL) based on a linear active disturbance rejection controller (LADRC) is proposed to largely improve this tradeoff, thus simultaneously enhancing the filtering capability and dynamic response. With LADRC adopted in PLL structures (LADRC-PLL), good dynamic response and disturbance rejection capability can be achieved. To pursue further improvements in both filtering capability and dynamic response, the NLADRC-PLL derived from the LADRC-PLL is proposed, which adjusts the controller gains adaptively according to the disturbance. It is commonly known that there are stability problems for PLLs with adaptive controller gains, which are highly nonlinear systems. Thanks to the great robustness of LADRC structures, the stability of the proposed NLADRC-PLL is certified by the second method of Lyapunov in the nonlinear model. By comparing its PLL performance with the existing PLL algorithms, the superiority of the NLADRC-PLL in both filtering capability and dynamic response is verified through experimental results.

Apart from the basic renewable generation function, the multi-functional grid-connected inverter (MFGCI) derived from traditional grid-connected inverter could provide ancillary power quality enhancement functions for the interfaced grid. However, with multiple functions integrated into one control system, the control scheme of MFGCI would become inevitably more complicated and vulnerable to the disturbance in distribution networks. Here, a control scheme of MFGCI for rejecting both the steady-state and dynamic-state disturbance is designed. The proposed control scheme includes an inner loop controller and an improved reference current generation algorithm, which constitutes an overall MFGCI control scheme. To reject the steady-state disturbance which would distort the output current of MFGCI, the inner loop controller by combining the linear adaptive disturbance rejection controller (LADRC) and repetitive controller (RC) is designed. While for the dynamic-state disturbance which would amplify the output current of MFGCI or even destroy the stability of the system, an improved reference current generation algorithm with enhanced dynamic disturbance rejection capability is proposed. The proposed MFGCI could provide overall improvements in both dynamic- and steady-state disturbance rejection capability. Finally, some experiments are conducted to verify the superiority of the proposed MFGCI.

In the paper, the H∞ repetitive current control scheme based on active damping along with the design method is proposed for three-phase grid-connected inverters with inductor-capacitor-inductor (LCL) filters. The control scheme aims to reduce the harmonic distortion of the output currents and achieve better efficiency. The design method introduces capacitor-current-feedback active damping into the H∞ controller design process by proposing an equivalent controlled plant. Additionally, based on the discrete model of the controlled plant with variable computation delay, the algebraic expression of the stable region for the feedback coefficient and the computation delay is obtained to avoid system instability caused by the digital control delay. Finally, the stability criterion is proposed to evaluate the stability of the discrete control system with the H∞ repetitive current control scheme. The theoretical analysis and experimental results prove that the control scheme presented in this paper not only can reject the harmonics of output currents, but is robust under the variation of the grid-impedance.